In a measuring cup, combine soy sauce, water, chopped garlic, lemon juice, and black pepper. Put the sliced beef in a ziploc bag or a bowl(with cover). Pour the marinade all over the beef. Marinate for 2-3 hours before cooking.
Heat up oil in a wok/pan. Sauté the beef with the marinade. Stir then cover to tenderize the beef. About 10-12 minutes, stirring occasionally.
Stir in the sliced onions.
Cover to continue cooking and tenderizing the meat. About 5 minutes or so.
Stir in the spinach. The leaves will wilt fast.
Stir in the sliced mushrooms. This will take about 2 minutes or so.
VOILA!